Thermodynamically, most metals are stable only under reducing conditions and corrode upon exposure to an oxidizing ambient. According to National Association of Corrosion Engineers, it costs the U.S. households, businesses, and government agencies $300 billion annually to address the problems caused by corrosion. Metals producers combat this problem using various methods. Steel producers use various organic and inorganic coatings to protect cold-rolled steel (CRS) sheets from corrosion during shipment and storage. Some coatings are designed to control the electrochemistry of corrosion, while others such as conversion and organic coatings create physical barriers to retard the corrosion rate in an oxidizing environment.
Traditionally, conversion coatings are produced by exposing CRS to phosphoric acid (phosphating) or chromic acid (chromating) or both. The latter provides more effective corrosion protection. However, concerns for the toxicity of chromium salts have generated considerable interest in developing chrome-free conversion coatings. Most new developments are based on organic coatings, which also protect steel from fingerprints that act as seats for corrosion.
Of the various chrome-free conversion coatings developed, perhaps the silane-coupling agents have received the most attention. These reagents are designed, due to their unique molecular structure, to bond strongly to metal substrates on one side and to organic topcoats on the other. Many conversion coatings formed by silane coupling agents are comparable or better than chrome conversion coatings and provide excellent seats for paint adhesion.
Van Ooij, et al. (U.S. Pat. No. 5,108,793 and No. 5,200,275) disclosed methods of rinsing a steel sheet for about 30 seconds in an alkaline solution containing 50 mM sodium silicate and 5 mM Ba(NO3)2, or Ca(NO3)2, or Sr(NO3)2 at an elevated temperature (>45° C.) and pH (<10). The steel sheet is then dried to form a relatively insoluble silicate coating, which is subsequently rinsed with a solution containing 0.5˜5.0% silane by volume.
Van Ooij, et al. (U.S. Pat. No. 5,292,549, No. 5,433,976 and No. 5,750,197) also disclosed a method of using both a crosslinking silane and a functionalized silane to treat a metallic substrate. The crosslinking silanes (or multifunctional silanes) are those that have silane groups at both ends of an alkyl chain. An example is 1,2-bis(triethoxysilyl)ethane (BTSE). Crosslinking silanes bond strongly to an inorganic substrate, while functionalized silanes adsorb on the top of the crosslinking silanes that have been adsorbed on the surface. In another U.S. patent, van Ooij, et al. (U.S. Pat. No. 5,759,629) disclosed a method of treating galvanized steel with vinyl silane (VS). It provides a high degree of paint adhesion and prevents delamination and underpaint corrosion.
Silanes adsorb on a metallic substrate (M) via hydrogen bonds initially, which can subsequently be transformed to covalent bonds (M-O—Si). The adsorbed silanes undergo cross-linking polymerization via siloxane bonding (Si—O—Si) between neighboring molecules. Both these bonding mechanisms contribute to large negative free energies of adsorption, which may contribute to the formation of robust surface coatings that can provide a strong barrier effect to corrosion.
In the instant invention, alkanethiols are adsorbed on galvanized CRS sheets to increase their resistance to corrosion. The thiol group (—SH) can react strongly with a metallic substrate (M) in an aqueous medium and form strong metal-sulfur (M-S) covalent bonds, while the alkyl chains can be associated with each other to form a close-packed monolayer of hydrocarbon chains. The attraction between hydrocarbon chains is often referred to as hydrophobic bonding. A monolayer of surfactant formed by the hydrophobic bonding mechanism is known as self-assembled monolayer (SAM). Both the covalent bonding and hydrophobic bonding mechanisms contribute to the formation of robust surface coatings that can provide a barrier effect. In general, the longer the alkane hydrocarbon chain length is, the stronger is the barrier effect to corrosion. When methyl (CH3) group is the terminal group of the hydrocarbon chain, the coated surface becomes hydrophobic, which should help prevent fingerprint formation. By substituting the methyl group with other functional groups such as —OH, —NH2, —COOH, etc., one can modify the affinity of the coated surface to a top coat. By using a polymerizable terminal group, one can further increase the barrier effect.
Zamborini and Crooks (1998) studied the ability of n-alkanethiol SAMs to protect gold from corrosion in aqueous bromide solutions. According to their voltammetry studies, corrosion resistance increased with increasing thickness for a given alkanethiol. With alkanethiols of approximately the same chain length, corrosion resistance varied with the terminal group, the corrosion resistance decreasing in the order of OH>COOH>CH3.
Scherer, et al. (1997) studied the corrosion of Cu(100) surface coated with alkanethiols with carbon numbers. (n) in the range of 8 to 16. The study was carried out in a 1-mM HCl solution using an in-situ scanning tunneling microscope (STM) and electrochemical techniques. They showed that the thiol coating inhibited the nucleation and growth of corrosion sites. When unprotected Cu(100) was allowed to oxidize, copper dissolved into solution layer-by-layer. When the surface was coated with a thiol, corrosion started as pits.
Azzaroni, et al. (2000) reported that the SAM of alkanethiol (n=12) hindered copper oxide formation and copper dissolution in electrolyte solutions containing chloride anions. They found that the corrosion inhibition varied with the electrode potential and the concentration of the aggressive anions.
Jennings, et al. (1996) coated silicon wafers with copper films, which were then coated with SAMs of alkanethiols. They showed that the monolayer films provided a barrier against the penetration of water and, thereby, increased corrosion resistance. In general, corrosion rate increased with increasing film thickness, which in turn varied with chain length. For example, the thickness was 1 nm when an alkanethiol with eight carbons (C-8) was used, and 3 nm when C-22 thiol was used. As a means of further increasing the thickness, the copper film was first coated with mercaptoalcohols (HS(CH2)nOH) with n=11 and 22 and then coated with an alkyltrichlorosilane (CH3(CH2)17SiCl3). However, the bilayer films were not effective in improving corrosion resistance.
Nozawa (1997, 1999), on the other hand, showed that bilayer coatings greatly increased corrosion resistance of iron. A monolayer coating of iron with 1-octadecanethiol (ODT) increased the protective efficiency by 76.3% as measured from impedance measurements in a 0.5 M NaCl solution. The iron surface was coated first by 11-mercapto-1-undecanol (MUO) and subsequently by triethoxyoctylsilane to achieve an 88.0% increased in protective efficiency. When the MUO-coated surface was coated again with 1,2-bis(triethoxysilyl)ethane (BTSE) and subsequently with 5×10−4 M of triethoxyoctadecylsilane, the efficiency was increased by 98.1%. Teneichi, et al. (2001) modified the MUO SAM with alkylisocynate (CnH2n+1 NCO) for the protection of copper in an aerated solution of 0.5 M Na2SO4. Protective efficiencies increased by 94.7 and 95.4% with octyl and octadecylisocyanate, respectively.
Halko, et al. (U.S. Pat. No. 6,102,521) disclosed a technique of treating a gold-plated orifice of ink-jet pen with thiol-type SAMs to control the wettability of the surface. Such treatment helped reduce the accumulation of residual ink and, thereby, inhibited corrosion and contamination of the plate.
Enick and Beckman (U.S. Pat. No. 6,183,815) disclosed a method for coating a metal surface with an amide thiol to increase corrosion resistance. The coating reagents with a general formula, F(CF2)mCONH(CH2)nSH, where n and m vary in the range of 2-20, were effective for protecting various metals such as gold, silver, nickel, copper, brass, tin, iron, etc., but not for aluminum and its alloys.
King, et al. (U.S. Pat. No. 5,487,792) used SAMs of 12-mercaptododecanoic acid to create a barrier effect and improve adhesion. The organized molecular assembly was impervious to water, alkali and other corrosive substances, and improved the adhesion of poly(methyl methacrylate) on silvered mirror surface.
Crottty, et al. (PCT WO 02/072283 A1) disclosed a method of treating metals, particularly aluminum or aluminum alloys with a solution comprising a mercapto-substituted silane and then baking the metal to cure the coating. The authors stated, however, that their invention does not include zinc or zinc plated surfaces.
Although it is well know that SAMs of alkanethiols are effective for protecting various metals from corrosion, no prior art discussed above is designed to protect galvanized CRS sheets. The instant invention is effective with galvanized and electrogalvanized (EG) steels even when they are coated further with phosphate and/or polymeric resins.